Optical information recording media can be classified roughly into read-only, write-once and rentable media. Examples of read-only optical information recording media include compact disks (CDs) on which audio data are recorded and laser disks (LDs) on which video data are recorded. Write-once optical information recording media are used for storing document files and still-picture files, and rentable optical information recording media are used to store data files in personal computers. The structure of such optical disks is that of a transparent resin substrate of 1.2 mm thickness provided with an information layer that is protected with an overcoat layer, or that of two transparent resin substrates of 1.2 mm thickness of which one or both are provided with an information recording layer and that are laminated together.
Large-capacity digital versatile disks (DVDs) on which not only audio data but also video data can be stored are becoming widespread. In order to record data at larger densities, it is necessary to make the wavelength of the laser light shorter, and to increase the numerical aperture (NA) of the objective lens. However, shorter wavelengths and greater NA lead to a narrower tolerance of the angle of inclination (tilt) of the disk with respect to the irradiation direction of the laser light. In order to increase the tolerance for tilt, it is advantageous to make the substrate thinner on the side where the laser light is irradiated. For example, under the present DVD standard, the wavelength of the laser light is 650 nm, the NA is 0.60, and the substrate thickness is 0.6 mm. The mechanical strength of resin substrates of 0.6 mm thickness is low, so that in DVDs, two substrates are laminated together, and an information recording layer is sandwiched between the two substrates.
It is expected that with the start of terrestrial digital broadcasts, the need for optical disks with capacities of about 25 GB will grow even larger. To realize capacities of that order with optical disks that are of approximately the same size as CDs and DVDs, it has been suggested to set the wavelength of the laser light to about 400 nm and the numerical aperture of the objective lens to about 0.85. In that case, when the thickness of the substrate on the side on which laser light is irradiated is set to about 0.1 mm, then a tolerance for tilt can be ensured that is about the same as with current DVDs.
When the thickness of one substrate is set to 0.1 mm, then, in order to maintain compatibility with DVDs, the thickness of the other substrate must be set to 1.1 mm. Increasing the asymmetry of the two substrates leads to the problem of deformation of the optical disks brought about by changes in environmental conditions, in particular changes in the humidity. This is because the recording/reproducing characteristics are affected when the tilt becomes large due to deformation of an optical disk.